This invention is concerned with printing plates and their preparation. In one aspect, the invention relates to the formation of a lithographic surface using hydrolyzed polyvinyl acetate and hydrolyzed tetraethyl orthosilicate. Another aspect of this invention relates to lithographic plates and their formation by diffusion transfer.
It is known to obtain lithographic printing plates by using various surfaces on suitable supports for lithographic plates which are intended for a relatively short press life. Where only a few hundred copies are required, or even up to about 1,000 copies, it is quite common to use a paper printing plate. However, it has been desirable to improve the lithographic coating used on the paper surface in order to decrease scumming in the non-image areas, improve wearability on the press, reduce the tendency for blinding, etc.
Perkins U.S. Pat. No. 3,055,295 issued Sept. 25, 1962, describes a planographic printing plate having thereon a lithographic coating comprising a polymeric material and a mixture of pigments. The polymeric material requires a cross-linking agent such as dimethylol urea in order to provide a satisfactory lithographic layer. However, it has been desirable to obtain a lithographic surface which is self-hardening.
In a particularly useful method for providing a printing plate, a silver image is obtained by the photographic diffusion transfer process. A silver precipitating or nucleated surface is provided on a hydrophilic surface on which complexed silver halide is transferred from a developing silver halide emulsion. Hepher et al., U.S. Pat. No. 3,161,508, describes the preparation of an aluminum lithographic printing plate by diffusion transfer and discloses treating a silver image obtained by diffusion transfer to improve its affinity for greasy printing ink. However, the formation of a paper lithographic plate using conventional paper supports having thereon a hydrophilic surface such as a layer of a crosslinked polymeric binder has not been entirely satisfactory due to problems of slow inkup, blinding, etc.
Thurlow, U.S. Pat. No. 3,083,639, issued Apr. 2, 1963, describes a planographic printing plate having a paper support with a planographic layer thereon comprising an essentially non-aqueous solution containing silicate esters and their reaction products. A coating of hydrolyzed ethyl silicate in a solvent solution containing methyl ethyl ketone is coated on a support and, before dryin, can occur, a second solution containing alkaline agents; such as, ammonium, sodium, or barium hydroxide, is coated over the initial acidic layer. When the first coating is allowed to dry, it results in a hydrophobic layer.
Bechtold, U.S. Pat. No. 2,404,357, issued July 23, 1947, discloses an abrasion-resistant coating for methyl methacrylate polymer comprising partially hydrolyzed polyvinyl acetate and hydrolyzed ethyl silicate. The coating is heated at an elevated temperature (i.e. about 150.degree.C) to cure the coating and bond the coating to the surface of methyl methacrylate polymer. However, the cured coating is hydrophobic.
Neither the Thurlow nor the Bechtold coatings meet the need of a hydrophilic polymeric pigmented layer having a surface which would have a desirable hardness, degree of affinity for water and would provide a good bond to a silver image obtained by the photographic diffusion transfer process or to hydrophilic coatings such as gelatin photographic coatings.
Accordingly, it has been desirable to provide an improved lithographic surface for preparation of lithographic plates especially by diffusion transfer, which would be hydrophilic with respect to an oleophilic image area, have good reproducibility, and which can be used as a surface for various types of oleophilic images.